1. A Quick Intro to Java for C and C++ Programmers
Scott Hudson
HCI 631
Fall 1999

2. About this lecture
Will be assuming you are a fluent C or C++ programmer.
This is going to be a quick introduction to the language.
I'm not going to teach anything much about object-oriented programming.

3. Java is a programming language designed by James Gosling at Sun
What is Java
Java is a programming language designed by James Gosling at Sun
Designed to be "squirted" across a network into a device, the primary device right now being a web browser
Interpreted from byte codes (virtual machine approach)
Dynamically loaded
=> very portable (except...)

4. What is Java
"A pretty good object oriented language with C++ camouflage on"
Once was "The latest thing" and massively hyped

5. Why was there hype about Java?
At the right place at the right time (fills important need on the web)
Highly portable
Seen by lots of companies as a way to break the Microsoft monopoly
=> huge amounts of money involved
=> major corporate politics/warfare (Sun & Netscape vs. Microsoft)

6. Not much fundamentally new here
Beyond the Hype
Not much fundamentally new here
we have seen almost all the concepts in other languages before
But, its fairly well designed
more than can be said for a lot of things
Its better than C++ for many/most uses
It was built by someone who knew the "landscape" of programming languages and has some taste

7. Beyond the hype
In my opinion its the best current language with wide platform support (backing of many solid implementations)

8. Downloadable into a wide range of devices/platforms
Goals
Downloadable into a wide range of devices/platforms
Good OO language to replace C++
Relatively small/simple/clean language
Camouflaged as C++ for sneak attack when C++ was the dominant language
C++ programmers will feel at home
Lots of checking and safety

9. Major differences from C++
Generally simplified
No pointers
Actually everything is a pointer (reference), but their are no dangling pointers!
No pointer dereferencing (where you used to say "->" you now always use ".")
Garbage collection added
Big big win

10. What's been added from C (all in C++)
O-O concepts & information hiding
Stronger typing
Real constants (not just macros)
Overloading (two functions with the same name, but different parameter types)
New comment style ("//" to eol)

11. What's been added beyond C++
More checking
Exceptions (they were in C++, but..)
(Limited) signature based typing ("interfaces" -- a big win)
Runtime type identification (plus other reflection capabilities)
Built in threads and synchronization

12. What's been added beyond C++ (cont.)
Packages (collections of classes in a namespace)
Dynamic runtime loading
Strings (as objects)
A root class (Object)
Unicode characters (16 bit chars)
64 bit longs, bytes, booleans
(literals true and false)

13. What's been added beyond C++ (cont.)
A (small) container library (hashtables, etc.)
Special "doc" comments /** ... */ + post-processing to produce hyperlinked API documentation
html + in the comments

14. Things that are missing (but we probably won't miss):
Virtual functions (everything is virtual!)
Virtual base classes
Virtual destructors
Implicit constructor invocation (and other constructor/destructor weirdness)

15. Things that are missing (but we probably won't miss):
Arcane type conversion rules (and generally loose typing)
16 different meanings for const
extern (now have strong load-time checking)
->* operator (member pointers)
:: (scope operator)

16. Things that are missing (but we probably won't miss):
void *
Null terminated strings
Declaration vs. definition (always done together, need not declare before use)
(Brain dead) multiple inheritance
Templates

17. Things that are missing (but we probably won't miss):
Operator functions
Macros, include files, and conditional compilation (the preprocessor)

18. Other things that are missing (that we might miss):
Independent functions
Pointers to functions
Default parameters
Unsigned types
Bare metal performance(?)

19. Plenty fast for interactive apps
So is it too slow?
Can get roughly 4x of C code performance (on many things you care about) using JIT compilers
Plenty fast for interactive apps
Spend a lot of time in native drawing code anyway
Faster development time => more optimization time => faster code!

20. More details on differences with C++ Lexical level
Designed to look almost exactly like C++ (which is almost identical to C)
Minor differences, but not worth mentioning

21. More details on differences with C++ Syntactic level
Designed to look a lot like C++
which was designed as a superset of C
but cleaned up most egregious parts
Changes:
inheritance syntax
initialization in constructors
split definition vs. declaration (and scope operator)
protection declaration

22. More details on differences with C++ Control Flow
All C/C++ flow control constructs except goto
added labeled break and continue to handle the few remaining legit uses
if/else for while do/while switch/case/break
return break continue
throw try/catch/finally to support exceptions

23. More details on differences with C++ Control Flow (cont.)
Now have real booleans
can't test integers and pointers directly
"if (ptr)" must be "if (ptr != null)"

24. Classes
The major construct in Java (like most object oriented languages) is the class.
Classes are a type definition: They define the data and operations of an object.
You can create several instances of objects; several things with that data and those operations

25. Classes
For C programmers you can think of this for now as a struct with the functions operating on the struct "inside" the struct.
For C++ programmers you can think of this as a class.

26. class point_list {
int x;
int y;
point_list next;
double distance_to(point_list other) {
double dx, dy;
dx = (double)(x - other.x);
dy = (double)(y - other.y);
return Math.sqrt(dx*dx + dy*dy); }
double distance_to_next()
if (next == null)
return 0;
else
return distance_to(next);

27. Three instance variables Two methods
What have we got here.
Three instance variables
C++ == fields
Two methods
C++ == member functions
Note: There are no functions in Java, just methods (notice that we had to say Math.sqrt() to get a square root).

28. Except... we have some problems here what are they? (p, v, c, after)
What have we got here.
If we had two points: pt1 and pt2, then we could compute the distance between them as
pt1.distance_to(pt2)
Except... we have some problems here what are they? (p, v, c, after)

29. Problem #1: self containment
Looks like a point_list contains a point_list
class point_list {...
point_list next;
But, recall everything is a pointer, so this is a pointer (reference) to a point_list, so we are ok

30. Like C++ Two forms of data: primitive and objects
Primitive types:
byte 8 bit signed integer
short 16 bit signed integer
int 32 bit signed integer
long 64 bit signed integer
boolean true of false
char 16 bit unicode character
float single precision
double double precision

31. If its not a primitive its an object
Note: this includes strings & arrays
Declaring instance variables of object types actually declares references to objects of that class
null is a possible value
need to be initialized to refer to object
Referenced with “.” instead of “->”
(back to problems)

32. Problem #2: Visibility
Classes also provide information hiding and the default is to not allow full access to instance variables or methods.
So a better version would be...

33. public class point_list {
protected int x;
protected int y;
protected point_list next;
public double distance_to(point_list other) {
double dx, dy;
dx = (double)(x - other.x);
dy = (double)(y - other.y);
return Math.sqrt(dx*dx + dy*dy); }
public double distance_to_next()
if (next == null)
return 0;
else
return distance_to(next);

34. Specifying protection Members can be:
public
protected
private
<unlabeled>
Accessible to all
Accessible to classes within the same package and subclasses
Accessible only within the class itself
Inside package: protected
Outside package: private
(no subclass access outside package).

35. Specifying protection Classes can be:
public, unlabeled, or private
Almost all classes are public (anybody can use them)
Can have private or local classes
only accessible in package
rare
can protect constructors instead

36. Point of style
Its a good idea for all but the simplest classes to not have any public instance variables.
Instead have a protected variable with read and write access methods.
This lets you change your mind later without breaking all the code that uses yours.

37. Point of style protected int _x; Seems tedious, but…
public int x() {return _x;}
public void set_x(int val)
{_x = val;}
Seems tedious, but…
This has saved my sorry butt many times! (back to problems)

38. Problem #3: no constructor
Haven't provided a way to create any useful objects of this type
Java initializes instance variables:
int et al. 0
boolean false
float, double 0.0
char ‘\0’
Object null

39. You can also explicitly initialize instance vars
Initialization
You can also explicitly initialize instance vars
protected int x = 0; protected int y = 0; protected point_list next = null;

40. Declaring constructors
Typically, you want provide a constructor.
If you don't provide a constructor, one (that does nothing and takes no parameters) will be provided for you.
Constructor looks like a method with the same name as the class but no return type:

41. Declaring constructors
public point_list(
int xv,
int yv,
point_list nxt) {
x = xv; y = yv; next = nxt; }

42. Declaring multiple constructors
Often want to provide several constructors
Java does not do default parameters
public point_list(int xv, int yv)
{ this(xv,yv, null); }
public point_list()
{ this(0,0); }

43. Declaring multiple constructors
Notice that we can "chain" constructors together using "this()".
We can also invoke the super class constructor using "super()".
Must be the first thing in the constructor.
(back to problems)

44. All parameters are pass-by-value.
Some misc. basics
All parameters are pass-by-value.
However, for all object types we are passing references by values, so...
Can return at most one value.
Arithmetic, etc. operations (precedence, etc.) are the same as C/C++, except...

45. Arithmetic, etc. operations are the same as C/C++, except...
Some misc. basics
Arithmetic, etc. operations are the same as C/C++, except...
Use of comma operator is limited to for statements.
Can apply bitwise operations to booleans (but not mixed int/boolean) to get non-shortcutting operations.
== and != mean "refer to the same object".

46. Like C++ can declare variables anywhere a statement is legal
Some misc. basics
Like C++ can declare variables anywhere a statement is legal
including inside parens of for statements
Same scoping rules
scope limited to enclosing block: {... }
Don't have to declare before use.

47. Hierarchical Types and Inheritance
The big wins in object-oriented programming come from hierarchical typing and inheritance.

48. Hierarchical Types and Inheritance
Two things of importance:
Inheritance: the ability to derive the implementation of something from some existing code (AKA getting someone else to do the work for you).
Substitutability: the ability to write code that doesn't care about exactly what type it operates over (so you can substitute related but different types).

49. Inheritance
In Java (like most OO languages) you can base the implementation of a class on another class.
Basically, you take what is in the other class (the base or superclass) and then extend it to do new and/or different things.
The new class (the derived class or subclass) preserves the API of the superclass

50. The new class preserves the API of the superclass
Inheritance
The new class preserves the API of the superclass
so it does everything the superclass did
Consequently, an instance of a subclass can be substituted for an instance of a superclass
a variable that references the superclass can safely be used to reference the subclass

51. Declaring inheritance in class declarations
Suppose we wanted to create a colored_point_list which keeps a color value with each point...

52. Declaring inheritance
public class colored_point_list extends point_list {
protected Color _pt_color;
public Color pt_color() {return _pt_color;}
public void set_pt_color(Color val) {_pt_color = val;}
public colored_point_list(
int xv, int yv, point_list nxt, Color clr)
{ super(xv,yv,nxt);
set_color(clr); }

53. Declaring inheritance
Note the extends clause
Gives the base class we are derived from
If there is no extends clause we automatically inherit from Object (the Java root class).
Java only supports single inheritance, so you only get to list one thing.

54. Declaring inheritance
Note also that we call the superclass constructor (might as well let them do the work here) using the special form:
"super(...);"
Like "this()" this must be the first thing in the constructor.

55. Get substitutability
If we have a piece of code which declares a variable of type point_list, we can safely let it operate on an object of type colored_point_list.

56. Another (different) use of “Super”
Can use “super” to refer to a method in the superclass that has been overridden by the subclass
public class grid8_point_list extends point_list {
public void set_x(int val) {
int rounded = (val / 8)*8 + ((val%8 >= 4)?1:0);
super.set_x(rounded); }
...

57. Boy it’s a good thing we put that set_x() in there
Notes on this example
public class grid8_point_list extends point_list {
public void set_x(int val) {
int rounded = (val / 8)*8 + ((val%8 >= 4)?1:0);
super.set_x(rounded); }
...
Boy it’s a good thing we put that set_x() in there
We call set_x() don’t set x directly

58. This
Sometimes you need to refer to the object itself in one of its methods (pass it somewhere).
Reserved word this is used for this purpose. As in:
manager.put_in_table(this);
Note that instance variable references such as "_x" above are really shorthand for "this._x".

59. Its not necessary to completely define a class
Abstract classes
Its not necessary to completely define a class
This is useful to define the API to something without requiring a particular implementation (see also interfaces).
This is useful also if you have a lot of common functionality to put in a base class but details (i.e., implementation of particular methods) have to be provided by various subclasses.

60. (Must) declare the class "abstract"
Abstract classes
(Must) declare the class "abstract"
Declare each missing method “abstract” and put a semicolon where the body would be.
abstract public class stack {
abstract public void push(Object obj);
abstract public Object pop();
abstract public boolean empty(); }

61. Abstract classes
Can't instantiate an abstract class (but can declare variables).

62. Basically interfaces define an API, but no implementation
Syntactically an interface declaration looks a lot like a class declaration (replace "class" with "interface").
But, no variables and methods don't have bodies.
Basically interfaces define an API, but no implementation

63. Interfaces are used as a "promise".
Class can say: "implements”interface_name" after extends clause
This means that the class promises to implement the API of the interface.
It doesn't say anything about how its going to do it, just that it will
The compiler checks that it does.

64. Interfaces
Can inherit from (“implement”) multiple interfaces.

65. Substitutability for interfaces
Can declare variables and parameters with interface types
Variable can then refer to object of any type that implements the interface
Has promised to implement the API (& compiler has checked) so this is completely type safe!
Gives you a lot of flexibility to reimplement, etc.
Advice: use this whenever you can

66. Classes are organized into collections of related things
Packages
Classes are organized into collections of related things
what constitutes one package is entirely up to you.
Each source file belongs to a particular package
listed at the top with the package declaration: package sub_arctic.lib;

67. Packages
Note: its possible to leave the package declaration out which indicates the special "unnamed package”
don't do this! it just gets confusing to everyone concerned

68. Package names have multiple parts (‘subpackages”) separated by "."
java.awt and java.awt.image
But, these don't have a special relationship between them
Packages are primarily to segregate the name space
class names must be unique within the package, but not across all packages

69. Also have an effect on protection rules
Packages
Also have an effect on protection rules
recall: protected members can be accessed by classes in same package

70. You can use package names to disambiguate types.
Imports
You can use package names to disambiguate types.
Recall we had an example that used the class Color.
Comes from the library package java.awt and its full name is java.awt.Color.
If you get tired of writing out the full name, you can "import" the class: import java.awt.Color;

71. Imports
If you get two classes with the same name from different packages you use full names to disambiguate.
Since compiler knows how to find packages, this eliminates the need for #includes
you can also import from compiled code without the source!)

72. Packages
java.lang.* is imported automatically.

73. Instance variables belong to (have a copy in) each instance.
Static members
Instance variables belong to (have a copy in) each instance.
Can also declare variables that belong to the whole class.
Do this with static:
class a_class {
protected static int count = 0;
public a_class() {count++;} }

74. There is only one copy of the static variable
Static members
There is only one copy of the static variable
belongs to the class (shared by all instances).
You can also have static methods.
These can be invoked without an instance (using the class name): Math.sqrt(3.1415d)

75. Initialization of statics
You can use conventional initializers (after =)
Executed when the class is loaded
Also have special static initialization blocks (outside of methods):
static {... some code ...}
Also executed when class is loaded
Java loader takes care of loading classes you depend on first

76. You can declare classes, methods, and variables final.
Final members
You can declare classes, methods, and variables final.
For variables this means they can't be assigned to after initialization (AKA constants):
public static final double PI2 = Math.PI*Math.PI;
For methods, this means you cannot override the method in subclasses.
For classes this means all the methods are final.

77. Final members
You can declare final variables in interfaces
Java idiom: to collect a bunch of reusable constants together put them in an interface and "implement" them in the classes where they are used.

78. In Java, strings are objects of class String
not array's of char, although there are operations to get back and forth
Special literal e.g: "abc" represents a String object.
Character escapes such as \n similar to C.

79. "+" is used for string concatenation, and is treated specially
Strings
Strings are immutable
operations create new strings, don't modify existing ones.
"+" is used for string concatenation, and is treated specially
Any + with at least one String operand is treated as string concatenation
The other operand is converted to a string (this also works for String parameters)

80. For primitive types there is a standard (and obvious) conversion
Conversion to Strings
For primitive types there is a standard (and obvious) conversion
For Objects, the toString() method is called
Object provides one (prints class name and unique id)
Or you can override

81. Arrays are also objects in Java
Can declare as "int foo[]" or "int[] foo"
both declare a variable which refers to an array of ints (initialized to null)

82. Array size is not part of the type
that gets established by the instance
int[] foo = new int[52];
...
foo = new int[42]
Each array object has a read-only field: length
foo.length == 42

83. Note that arrays are collections of Object references.
All set to null by default.
Can initialize arrays to refer to particular object with special form similar to C/C++
init[] foo = {3, 5, 9};
Object[] bar = {"one", new Stack(), new Integer()};

84. Arrays are hierarchically typed
Array types
Arrays are hierarchically typed
If sub is a subclass of base then sub[] is a subclass of base[]
So you could have:
base[ ] base_var = new sub[42];

85. Checking types at runtime
Can determine if an object qualifies as a particular type at runtime using expression: "obj instanceof aclass"
"null instanceof aclass" is always false.

86. Checking and converting types at runtime
Can convert to a sub- or super-class with a C style cast expression: "(aclass)expr"
Does runtime type check and throws an exception if this is not a legal conversion (i.e., object being cast is not of that type or a subclass thereof).

87. Output to stdout
java.lang.System has lots of useful stuff for accessing the local environment including:
System.out, System.err, and System.in (for stdout, stderr, and stdin).
Two operations of particular interest: System.out.print() and System.out.println() print anything convertible to a string with or without a newline.

88. Threads
Threads are built into the language
Concurrent threads can greatly simplify program structure and design for a number of things
They also make testing and debugging nearly impossible
So unless your programs all work without testing (and/or there is no reasonable way around it), I advise: Just say no!

89. Threads
They also make testing and debugging nearly impossible
So unless your programs all work without testing (and/or there is no reasonable way around it), I advise: Just say no!

90. Good luck...
... and, as always, if you or any of your programming team are caught or killed, I will disavow that I ever taught you anything about Java.